3. How to invoke PSGML

PSGML defines major modes called sgml-mode and xml-mode.
Files with extensions `.sgml', `.sgm' or `.dtd' will
automatically be edited in SGML mode. To edit some other file in SGML
mode, type M-x sgml-mode RET after finding the file.
To edit XML files, type M-x xml-mode RET.

If you can modify the file you can add a Local Variables list
(see section `Local Variables in Files' in The Emacs Editor) to the end of the file. This can make Emacs
automatically set sgml mode and user options when the file is loaded.
The simplest Local Variables list would look like:

<!--
Local Variables:
mode: sgml
End:
-->

You can also put a line at the top of the file to tell emacs to use sgml
mode:

<!-- -*- sgml -*- -->

For XML replace `sgml' with `xml' in the above examples.
But remember that you can't have a comment before the SGML
declaration or the XML declaration.